    Japanese Patent Document 1: JP 58-137616 A    Japanese Patent Document 2: Japanese Utility Model Application Laid-open No. Sho 61-179414
Known conventional examples of a ball spline of this type are disclosed in JP 58-137616 A, Japanese Utility Model Application Laid-open No. Sho 61-179414, etc. Each of these ball splines is composed of a spline shaft equipped with a plurality of lines of ball rolling faces extending in the longitudinal direction, and a spline nut assembled to the spline shaft through the intermediation of a large number of balls and equipped with endless circulation paths for the balls, with the spline nut being freely movable around the spline shaft in the longitudinal direction thereof as the balls make endless circulation.
A ball spline is characterized not merely in that the spline nut can freely make a linear movement along the spline shaft but also in that mutual transmission of torque is possible between the spline shaft and the spline nut with respect to the circumferential direction of the spline shaft. In the ball spline as disclosed in Japanese Utility Model Application Laid-open No. Sho 61-179414, to allow transmission of larger torque between the spline nut and the spline shaft, longitudinally extending ridge portions are provided at three circumferential positions of the spline shaft, which has a substantially cylindrical sectional configuration, and at the base of each ridge portion, there are provided a pair of ball rolling faces on both sides of the ridge portion. In the ball spline as disclosed in JP 58-137616 A, the spline shaft itself has a substantially rectangular sectional configuration, and there are provided in both side surfaces thereof longitudinally extending wide groove portions, with a pair of ball rolling faces being provided with respect to the corners of these groove portions.